This invention relates to casing suspension systems and more particularly to a mudline suspension system having a load transfer mechanism and a latching mechanism to connect an inner casing hanger to an outer casing for suspending casing subsea.
The subsea exploration of oil and gas reserves includes suspending a plurality of concentric strings of casing within the bore of the well. To suspend the strings of casing using a mudline suspension system, a series of hangers to which the casing strings are connected are stacked and supported within a conductor casing located at the mudline. The conductor casing extends from the mudline to a surface wellhead and blow-out preventer and through which the drilling equipment is raised and lowered during the drilling and completion of the well. In the installation of successive casing strings within the well, a hanger assembly suspending the casing string is lowered through the conductor casing for suspension within the outer casing. Upon reaching location, the hanger assembly must be properly located within the outer casing and the weight of the casing string transferred to the outer casing.
The load transfer mechanism of the inner casing hanger must contract to a diameter that will allow the inner casing hanger to be lowered through the string of conductor casing. Once the inner casing hanger reaches a predetermined location within the previously installed outer casing at the mudline, the load transfer mechanism expands and forms a connection between the inner hanger and outer casing. Further, the load transfer mechanism cannot block fluid flow through the annulus between the two casing strings, particularly during the cementing operation, and thus the suspension system must provide a flow-by area.
Various U.S. patents disclose load transfer and latching mechanisms. U.S. Pat. No. 4,730,851 discloses a hanger assembly suspended within an outer casing. The hanger assembly includes a hanger body with an expandable locking assembly comprised of a plurality of latching fingers secured at their lower ends about the hanger body by a split retainer ring. The split retainer ring permits limited pivotal movement of the latching fingers about their lower ends. The upper end of each latching finger has a hook which engages an upwardly facing shoulder on the inner profile of the outer casing. The free ends of the latching fingers are urged outwardly by a circular spring. The retainer and locking assembly are secured to the hanger body by a connecting shear ring which restrains relative axial movement of the locking assembly until the latching fingers hook on the inner profile of the outer casing whereby the shear ring is sheared by the weight of the inner casing string.
U.S. Pat. No. 3,741,589, in FIGS. 1-6, illustrates a hanger body having a reduced diameter midportion, a slightly larger diameter locking portion, and upper and lower stop shoulders. The lower stop shoulder includes a rim and a lip. A support ring, axially split along one side, is disposed around the reduced diameter portion. Ring 30 also includes a lower projecting lip which cooperates with the rim lip to initially hold the ring in the retracted position. The ring also includes lower guide segments having a cam surface. As the hanger is lowered into the casing, the camming surface retracts the ring such that the lips become disengaged. This allows the ring to move upwardly relative to the hanger body until the guide segments and support segments of the ring encounter the recesses in the wellhead and spring outwardly to engage those surfaces to support the hanger within the wellhead. The backup portion maintains the lock ring in the supporting position. FIGS. 7-11 disclose a latch release assembly which includes a carrier and an elongate releasing latch. The releasing latch rides in a slot in the carrier and pivots about the pin. A leaf spring is attached to the latch to bias the latch outwardly. As the hanger assembly is lowered through the blowout preventer and surface equipment, the rounded leading edge of the latch cams the latch inwardly. Upon the latch engaging the recesses in the wellhead, the casing weight shears the pins to permit the hanger body to continue to move downwardly and allow the suspension ring to expand into the recesses.
Other patents having load and latch mechanisms include U.S. Pat. Nos. 3,918,747; 4,232,889; 4,276,932; 4,295,665; 4,355,825; 4,422,507; 4,468,055; 4,509,594; 4,569,404; and 4,757,860.
Prior art load transfer mechanisms are formed of split, or colleted rings that expand into the outer casing by means of a radial sliding motion. This radial motion may also include an axial motion as well. Most prior art latching mechanisms expand virtually the same distance in the radial direction along the entire axial length of the split load transfer ring or portion of the ring. In some inner and outer casing assemblies, the annulus containing the load transfer mechanism in its contracted position is not much larger than the radial expansion required to form the connection to the outer casing. In such prior art designs, the load bearing "contact patch" between the load transfer mechanism and the inner casing may diminish during radial expansion to the extent that the load carrying capacity of the inner casing is limited by high contact stresses between the split load transfer ring and the inner casing as opposed to being limited by other factors. Also, the required flow-by area is typically fulfilled in a portion of the annulus vacated by the split load transfer ring. A fine balance must be struck between flow-by area, load carrying capacity (size of the contact patch), and strength of the inner hanger body itself. The radial translation required to connect the inner hanger and outer casing becomes a key to how well these factors are balanced.
The inner hanger assembly must also include a means for finding the proper location within the outer casing for locating the inner hanger and for activating the load transfer mechanism. In prior art designs, a profile is provided in the bore of the outer casing to provide the precise location for the inner casing hanger and to allow for the activation of the load transfer mechanism so that the inner casing hanger, and associated casing string, may be suspended from the outer casing. It is often the case that the previously installed outer casing, and its profile to be located and interfaced by the inner hanger, is contaminated with cement or other residue as a result of the drilling and cementing operations. When such contamination exists, it may not be possible for the inner casing hanger to "find location" and subsequently form the desired interface. "Finding location" and successively "latching in" is a critical operational step in mudline suspension systems. Prior art mechanisms only incorporate a single split ring or colleted ring which must expand into the recess of the profile in the outer casing.
The present invention overcomes these deficiencies of prior art mudline suspension systems.